The present invention relates to an embossing belt for paper machines for manufacturing an embossed paper web.
DE-195 48 747 discloses a paper machine for making creped tissue, which has a press comprising a shoe press roll, a counter roll and a suction roll, the counter roll forming a first press nip with the suction roll and a second extended press nip with the shoe press roll. A felt runs through the two press nips together with the paper web and then brings along the paper web to a Yankee cylinder, to which the paper web is transferred when the felt and the paper web pass round a transfer roll, which forms a non-compressing nip with the Yankee cylinder. Suction zones for dewatering the felt are available before and after the first press nip, the suction zone before the press nip being located inside the suction roll while the suction zone after the press nip is located in a side loop, in which the felt runs alone to meet again the paper web at the entry of the second press nip. Such a paper machine is inconvenient since the paper web is rewet by the wet felt before it reaches the Yankee cylinder.
U.S. Pat. No. 5,393,384 discloses a paper machine for producing a tissue web, which in the embodiment according to FIG. 6 comprises a non-compressible, water-impermeable belt, the underside of which conducts a paper web through a shoe press nip and from there to a Yankee cylinder, via a transfer roll which forms a nip with the Yankee cylinder. This impermeable belt has a smooth web-carrying surface which makes an adhesive water film form thereon as the belt passes through the press nip together with a press felt which has a non-smooth surface in contact with the paper web. As is known, a Yankee cylinder has a smooth surface. Since both the Yankee cylinder and the impermeable belt have smooth surfaces which the paper web is intended to contact, there is a risk that the paper web continues to adhere to the smooth surface of the impermeable belt after having passed the nip adjacent to the Yankee cylinder instead of being transferred, as desired, to the smooth surface of the dryer cylinder. Not even if large amounts of adhesive are applied to the circumferential surface of the dryer cylinder will it be possible to ensure that the paper web adheres to the Yankee cylinder.
U.S. Pat. No. 5,298,124 produces an excellent presentation of the tasks which a transfer belt in corporation with a press felt should perform in a satisfactory manner, as well as the properties and construction of such transfer belts which were then disclosed in Patent Publications U.S. Pat. Nos. 4,483,745; 4,976,821; 4,500,588; 5,002,638; 4,529,643 and CA-A-1,188,556.
The critical tasks, according to U.S. Pat. No. 5,298,124, of a transfer belt intended for cooperation with a press fabric comprise a) removing the paper web from the press fabric without causing instability problems; b) cooperating with the press fabric in one or more nips to ensure optimal dewatering and a high quality of the paper web; and c) transferring the paper web in a closed draw from a press in the press section to a paper-receiving wire or belt in the subsequent press (presses) in the press section, or to a pick-up wire in the dryer section.
U.S. Pat. No. 5,298,124 suggests a transfer belt for the press section in a paper machine having a specific design, and such a transfer belt is shown and described for operation in three paper machines with different press sections, the belt transferring the paper web from the press section to a dryer fabric which brings the transferred web to a dryer cylinder. In FIG. 2 of this publication, the press section has a shoe press nip as the last press nip.
The transfer belt disclosed in U.S. Pat. No. 5,298,124 for the press section of a paper machine has a web-contacting surface which is essentially impermeable to water and air and which has a pressure-responsive microscale topography. Under the action of the pressure in a press nip of the press section, the transfer belt is compressed such that the microscale roughness of said surface decreases, making the surface much smoother and allowing a thin, continuous film of water to be built up between the paper web and said surface. The thin, continuous film of water provides much stronger adhesive forces between the paper web and the transfer belt than between the paper web and the press fabric, so that the paper web may reliably follow the transfer belt as the paper web leaves the press nip. In this connection, the transfer belt expands in the direction of thickness and approaches its non-compressed state such that the film of liquid on said web-contacting surface breaks up.
Paper machines for manufacturing high bulk soft tissue are known from a plurality of patent specifications. As a rule, an embossing fabric or embossing felt is used, which together with the formed paper web runs through a press nip, in which the paper web as pressed into the embossing fabric and in this way obtains an embossed pattern on one side. Paper machines having such embossing fabrics and press nips are disclosed in U.S. Pat. No. 3,301,746, U.S. Pat. No. 3,537,954, U.S. Pat. No. 4,309,246, U.S. Pat. No. 4,533,437,
U.S. Pat. No. 5,569,358, U.S. Pat. No. 5,591,305 and WO 91/16493. A drawback of paper machines according to these publications is that the dewatering in the press nip is relatively low such that the dry solids content of the paper web is low as the paper web is transferred to the dryer cylinder, which results in a relatively low production rate of the paper machine.
U.S. Pat. No. 4,849,054 discloses a machine for manufacturing an embossed high bulk fibrous web without using a press nip. A roll, such as a transfer roll or a felt-carrying roll, forms a nip with an embossing fabric in a transfer point of the web where the embossing fabric runs round a vacuum tube which has a slot opening directed towards the transfer point. The nip is so wide that the web is not compressed when running therethrough. The suction of the vacuum tube via the narrow slot opening is sufficient so as not only to transfer the web to the embossing belt but also to conform the web to the embossing belt surface facing the web and having a three-dimensional pattern. Before the transfer point, the fibrous web has a speed which is higher than that of the embossing fabric. The roll carrying the web up to the non-compression nip has a smooth surface, and it is commonly known that in practice great problems are involved in the transfer of a fibrous web from a smooth surface to a fabric, said fibrous web being pre-pressed to a dry solids content of 30-50%.
U.S. Pat. No. 5,411,636 discloses the manufacture of tissue paper, in which the paper web is formed on a forming fabric, pre-pressed in a double-felted press nip and transferred to a coarse mesh fabric. When the paper web is supported by the coarse mesh fabric, it is subjected, in a suction zone, to vacuum, such that the paper web is sucked into the openings and depressions of the fabric, so that the paper web obtains an increased thickness and, thus, increased bulk. The coarse mesh fabric then carries the paper web to the dryer cylinder. The double-felted press nip implies that the dry solids content of the paper web after the press nip is relatively low, in fact 25-30%. Since no dewatering can be carried out in the nip adjacent to the dryer cylinder, the dry solids content of the paper web in the transfer to the drying cylinder is correspondingly low. Moreover, it is most uncertain whether the paper web is really transferred from the felt to the coarse mesh fabric.
An object of the present invention is to provide an improved embossing belt, by means of which it is possible to manufacture an embossed fibrous web having a high bulk and a high dry solids content before the Yankee cylinder, thereby achieving a high production rate at a reasonable cost, and in a reliable manner also transfer the embossed fibrous web to the Yankee cylinder.
The object is achieved by an embossing belt according to claim 1. Advantageous embodiments have the features stated in the dependent claims.
According to the invention, it has surprisingly been found that impermeability or essential impermeability is a very advantageous property of an embossing belt according to the invention if the impermeable embossing belt is also used to convey a pressed paper web to the transfer nip adjacent to a Yankee cylinder in the dryer section of the paper machine. Owing to this property, vapour which because of the heating of the Yankee cylinder forms in the depressions or pits in the embossing pattern, of water in the pits or depressions, can be pressurised, which presses the paper fibres which are also present in the pits or the depressions because of the pressing effect of the press section, such that these, in the nip of the Yankee cylinder, are pressed into the pits or depressions while at the same time the fibrous web parts that are positioned between the elevation of the embossing pattern and the Yankee cylinder become thinner, thereby achieving the desired embossing effect and a high bulk of the paper web.
The embossing effect and productivity can be increased if the embossing belt or a layer thereof intended for contacting the paper web is also given the capability of reversible compressibility, such that the embossing belt is compressed in the transfer nip adjacent to the Yankee cylinder. When the embossing belt then leaves the transfer nip while resuming its non-compressed state, vacuum is generated and contributes to the formation of water vapour, which in turn results in, on the one hand, easier separation of embossing belt and paper web after the transfer nip and, on the other, quicker drying of the paper web on the Yankee cylinder, i.e. a higher paper production capacity. The vacuum generating effect becomes greater the quicker said resumption occurs, i.e. the more elastic the reversible compressibility is.
The embossing pattern of the embossing belt according to the invention is of course selected according to the desired embossed pattern of the paper that is to be manufactured. The embossing pattern is regular across the embossing belt or, if the embossed pattern of the paper web should comprise a distinguished additional pattern, such as a picture or logotype, it has a regular basic pattern of depressions or pits and elevations, on which pattern the additional pattern is superposed. By regularity is not necessarily meant regularity over all directions of the embossing belt. For instance, if the paper is soft tissue which is to be creped, a closer, dominant transverse (transversely of the machine direction) pattern, gives an increased crepe effect compared with a longitudinal pattern of depressions and pits. The properties of the paper can thus be changed in the desired direction by means of the pattern.
The embossing pattern can be provided in some prior-art manner with regard to the material of the embossing belt or its surface layer intended to engage the paper web, such as by etching, calendering, laser treatment or embossing.
The drying effect on the paper web on the Yankee cylinder can also be affected by the closeness of the embossing pattern. Thus, fewer contact points between the Yankee cylinder and the paper web produces a reduced drying effect of the Yankee cylinder, but an increased drying effect of the hot air hood round the Yankee cylinder on the fluffier parts of the paper web which extend between the thinner contact points.